1. Field of the Invention
The present invention relates generally to a small twin spool gas turbine engine, and more specifically to a hollow rotor shaft.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine is a very efficient power plant and is used to power an aircraft such as a commercial or military aircraft or an unmanned aero vehicle (UAV). The PW4000 series engine made by Pratt & Whitney Aircraft Group is a large commercial turbofan engine with a dual-shaft (twin spool) and high bypass front fan. This engine produces 60,000 pounds of thrust and weighs 9,200 pounds dry. It is a very efficient engine. A twin spool engine has about two times the efficiency of a single spool engine and therefore the twin spool engine is used when efficiency is an important factor.
Recent developments in small unmanned aircraft, such as a UAV, have led to the use of small gas turbine engines to power these small aircraft. The more efficient the engine is in the UAV, the longer will be the loiter time. Small single spool gas turbine engines have been around for years to power small radio controlled planes or even a UAV such as a cruise missile or a video surveillance aircraft. With the demand for longer loiter times, a more fuel efficient gas turbine engine is desirable.
It has been common in the art of gas turbine engine design to scale down larger engines to the size needed. A small UAV only requires a few hundred pounds of thrust to power the aircraft. One major problem in the design of small gas turbine engines is scaling the larger engine down to the smaller size. As the size of the engine decreases, the rotational speed of the rotor shaft must increase in order to retain the high overall efficiency for a gas turbine engine. As the rotor shaft speed increases, the rotor dynamics becomes a major problem. A large gas turbine engine such as the PW4000 series engine cannot be scaled down below a certain size before the rotor dynamics and natural frequency problems start to cause problems. The low speed rotor shaft in the PW4000 series engine can be scaled down only so far before the bending mode (third mode) of the rotor shaft coincides with the natural frequency of the shaft. This is referred to in rotor dynamics as the critical speed. At the critical speed, a rotary shaft would break apart from the high bending stresses developed at the critical speed. The bending displacement would be infinite without adequate damping of the bearings. Engineers attempt to design the rotor shaft to operate below the critical speed so that these rotor dynamic problems do not occur.
In a twin spool gas turbine engine, a low pressure fan or compressor and a low pressure turbine are rotatably attached to the inner or low speed rotor shaft. A high pressure compressor and high pressure turbine are rotatably attached to the outer or high speed rotor shaft. Each shaft is rotatably supported by bearings on the shaft ends. The natural frequency of a rotating shaft is directly proportional to a ratio of the stiffness to the mass of the shaft. Thus, the natural frequency of the rotating shaft can be increased by either increasing the stiffness of the shaft, decreasing the mass of the shaft, or both.
Also, in the rotating shaft supported by bearings on both ends, as the length of the rotating shaft between bearings increases, the natural frequency decreases. If the shaft is lengthened, the natural frequency will be lowered. Thus, to produce a small gas turbine engine useful for a UAV or other small aircraft with a thrust of less than around 300 pounds, and to provide for a twin spool gas turbine engine in order to significantly improve the fuel efficiency of the engine, a new design is required for the inner or low pressure rotor shaft to make such an engine operational. The inner or low speed rotor shaft in a twin spool gas turbine engine requires a new design since the prior art twin spool engines cannot be scaled down to this level without the rotor dynamics problems occurring that limit the size of the shaft.
U.S. Pat. No. 5,454,222 issued to Dev on Oct. 3, 1995 and entitled SMALL GAS TURBINE ENGINE HAVING ENHANCED FUEL ECONOMY discloses a gas turbine engine having twin spools that has a smaller size and lesser weight than current turbine engines having the same power (at the time of the Dev invention) with a core engine having a diameter of about 0.35 meters (about 14 inches) that operates at about 54,000 rpm. As can be seen from the figures in the Dev patent, the inner or low speed rotor shaft is a straight solid shaft while the outer or high speed rotor shaft is solid but somewhat curved to conform to the radial compressor and turbine on the shaft. The twin spool engine of the Dev patent would be typical of the prior art twin spool gas turbine engines in that these engines cannot be scaled down any further because the rotor shafts would have to operate at higher speeds which would then produce the rotor dynamics problems discussed above. The limit of size for the Dev engine is reached.
It is an object of the present invention to provide for a rotor shaft that can operate at a higher rotational speed than the prior art rotor shafts without reaching the critical point for that shaft.
It is another object of the present invention to provide for a small twin spool gas turbine engine that can be scaled down to about 4 inches in diameter.
It is another object of the present invention to provide for a process for making a hollow rotor shaft that is capable of high speed rotation while not exceeding the critical speed for that shaft.